Continuing Education: TYMPANOMETRY

1 2222 TYMPANOMETRY is a non-behavioral test of middle ear function, which means it requires no voluntary response on the part of the client being tested. It can be routinely utilized by the HIS clinician in private practice, and should become a regular part of a client test battery. As health care professionals know, one cannot base conclusions on one single test. As math teachers always say, It takes two dots to make a line. In our field, air-bone gaps seen in Pure Tone Testing can be backed up by a quick, five-minute assessment of TYMPANOMETRY . The purpose of this article is to describe the principles behind commonly used TYMPANOMETRY , how it is done, and how to interpret the results. The general thrust here is to familiarize clinical practitioners, in the clearest way possible, with generally known and widely accepted procedures of TYMPANOMETRY . I. Why Do We Have MiddleEars in the First Place?Figure 1 (above) shows the middle ear is a closed space, filled with air.

2 One purpose of the middle ear is to change or transduce incoming sound waves into mechanical piston-like energy. The cochlea of the inner ear is filled with fluids called perilymph and endolymph. Perilymph, which fills the outer two boney labyrinths is similar to the fluid that surrounds the brain; namely, cerebral-spinal fluid. The inner membranous labyrinth is filled with endolymph, which has the opposite chemical composition. The job of the cochlea is to transduce fluid motion energy into electrical energy, because this is the language the brain understands. The Middle Ear Increases Sound PressureFigure 2 shows that the middle ear in-creases the pressure of airborne sound so that it can activate the fluid-filled cochlea. Airborne sound cannot otherwise activate a fluid-filled cochlea. Think of having your head under water in a swimming pool as The purpose of this article is to describe the principles behind commonly used TYMPANOMETRY , how it is done, and how to interpret the educationWritten by: Ted VenemaFigure 2.

3 The Middle Ear Increases Sound Pressure 3 Ways: 1. TM is larger than footplate of Stapes (17:1) 2. Leverage action of ossicles (Malleus is :1 longer than Incus) 3. Buckling action of TM (2:1) 1. 1. 2. Umbo TM at rest Buckling action 3. 2. Figure 1. The Middle ear is a closed space and thus, quiteinaccessible to scrutiny from the outside. Continuing Education: TYMPANOMETRYF igure 1. The Middle ear is a closed space and thus, quite inaccessible to scrutiny from the outside. Take the Quiz on page 30 to earn 1 Continuing Education Credit2323continued on page 24you try to hear someone speaking who is standing on the edge. You won t hear much because almost all of the airborne sound will bounce off the water. The same would happen if we didn t have middle ears. Almost all of the mechanical energy from the middle ear would bounce off from the cochlea. The middle ear increases sound pressure in three ways. First, the working surface area of the tympanic membrane (TM) is 17 times larger than the footplate of the stapes which sits inside the oval window, the entrance to the cochlea.

4 Pressure is force over an area. To appreciate this, push hard with your whole palm of your hand against your cheek and feel the pressure. Now push against your cheek with the same force using just your finger tip. You ll feel lots more pressure. It s the same reason why a sharp knife cuts through bread. In the middle ear, force upon the large TM area is converged onto a much smaller area of the stapes, and this increases the pressure by 17 times. Second, the middle ear ossicles are shaped the particular way that they are, so they can act like a lever. The malleus is times as long at the long process of the incus. This increases the pressure by a factor of :1. Third, the TM itself does not move as a whole in exactly the same way. When activated by airborne sound, it buckles, such that parts of it move more than other parts. This increases the pressure by a factor of 2 3 (above top) shows how these three pressure increases multiply together, and also how this translates into a decibel (dB) increase.

5 The total pressure increase (17 X X 2) works out to something close to 44:1. Readers may recall from past studies of sound that if sound pressure is increased by 10 times, there is a 20 dB increase; if the pressure increase is 100:1, there is a pressure increase of 40 dB. The 44:1 pressure increase offered by the middle ear is between 10:1 and 100:1, and it mathematically works out to an increase of somewhere between 30-35 dB. One might think then that the maximum conductive hearing loss (HL) would be between 30-35 dB HL. As we know however, a conductive HL due to otitis media (OM) or otosclerosis can easily be more than this. How? Any pathology that prevents the stapes from pushing into the oval window, and consequently bulging out the round window, will add even more dBs to the HL than the middle ear normally provides. Examples here could be otitis media with fluid in the middle ear space or otosclerosis. This is why conductive HL can often be greater than 30-35 dB HL.

6 Outer and Middle Ear Resonances and SpeechFigure 4 (above) shows that our outer and middle ears actually improve our hearing for the high-frequency consonants of speech. The middle ear ossicles resonate Figure 3. The middle ear must increase the pressure of air-borne sound because the cochlea is filled with fluid! 0 10 100 1000 10,000 100,000 1,000,000 120 100 80 60 40 20 0 Pressure dB SPL 44:1 33dB In Summary: 1. Eardrum Stapes size: 17:1 2. Ossicles leverage action: :1 3. Eardrum buckling action: X 2:1 44:1 This corresponds to an increase between 30-35 dB Figure 3. The middle ear must increase the pressure of air-borne sound because the cochlea is filled with fluid! 0 10 100 1000 10,000 100,000 1,000,000 120 100 80 60 40 20 0 Pressure dB SPL 44:1 33dB In Summary: 1. Eardrum Stapes size: 17:1 2. Ossicles leverage action: :1 3.

7 Eardrum buckling action: X 2:1 44:1 This corresponds to an increase between 30-35 dB + = Hz dB SPL 125 250 500 1000 2000 4000 8000 40 25 10 0 Note how important speech Hz s are emphasized 250 500 1000 2000 4000 8000 30 20 10 0 Total Ear Canal & Concha 250 500 1000 2000 4000 8000 30 20 10 0 Middle Ear Figure 4. The resonances of the Outer and Middle ears serve to create an equal loudness curve that shows our best hearing sensitivity is between 1000 to 4000 Hz. + = Hz dB SPL 125 250 500 1000 2000 4000 8000 40 25 10 0 Note how important speech Hz s are emphasized 250 500 1000 2000 4000 8000 30 20 10 0 Total Ear Canal & Concha 250 500 1000 2000 4000 8000 30 20 10 0 Middle Ear Figure 4. The resonances of the Outer and Middle ears serve to create an equal loudness curve that shows our best hearing sensitivity is between 1000 to 4000 Hz.

8 + = Hz dB SPL 125 250 500 1000 2000 4000 8000 40 25 10 0 Note how important speech Hz s are emphasized 250 500 1000 2000 4000 8000 30 20 10 0 Total Ear Canal & Concha 250 500 1000 2000 4000 8000 30 20 10 0 Middle Ear Figure 4. The resonances of the Outer and Middle ears serve to create an equal loudness curve that shows our best hearing sensitivity is between 1000 to 4000 Hz. 24best at around 2000 Hz, and the middle ear space has two other resonances of 750-900 Hz and 1200 Hz. The outer ear canal resonance falls roughly between 1500 and 4000 Hz. Together, the outer and middle ears thus serve to create the human hearing sensitivity curve, which shows our very best hearing sensitivity to be between 1000-4000 Hz. This all contributes to better hearing for speech. II. TYMPANOMETRY and the Middle EarFigure 5 shows that TYMPANOMETRY involves the use of a probe inserted into the ear canal with a tight seal, so that no air can leak out.

9 The assumption behind TYMPANOMETRY is that in order for the middle ear to be most efficient at passing incoming sounds through it, air pressure should be even on both sides of the TM. Contrary to common belief, TYMPANOMETRY does not determine how much the eardrum wiggles. The probe has three holes in it to provide: 1) a tiny speaker, 2) a tiny microphone, and 3) a way to change air pressure. The client can feel these air pressure changes during the test. During the air pressure changes, a steady low-frequency tone at 70 dB sound pressure level (SPL) is presented through the probe speaker, and the probe microphone picks up whatever sound bounces back off from the TM. If the least amount of sound bounces back off the TM when the air pressure in the outer ear canal is at regular room air pressure, this means the air pressure behind the TM is the same. In this way, TYMPANOMETRY measurement in the outer ear canal tells us about the middle ear air pressure behind the TM!

10 Why Does TYMPANOMETRY Typically Use a Low-Frequency Tone?With TYMPANOMETRY , we test the com-pliance of the middle ear by measuring the amount of low-frequency tone reflecting off the stiff middle ear as a function of air pressure changes. Compliance is the opposite or inverse of stiffness. TYMPANOMETRY uses a low-frequency tone because the middle ear is a stiffness dominated system. The middle ear system, which involves the TM and ossicular chain, is always stiff, but it is least stiff when the air pressure is even on both sides of the TM. The middle ear ossicles are tiny and therefore, do not have much mass. Stiffness is therefore the main source of opposition to the passage of sound through the middle ear. Stiffness opposes the passage of low frequencies and resonates with high frequencies, while mass opposes the passage of high frequencies and resonates with low frequencies. A low-frequency tone is used so that some sound will bounce off from the TM, even when the middle ear is least stiff.